This invention relates to truck beds and drop-in truck bed liners therefore, rear cargo areas of motor vehicles such as sport utility vehicles (SUVs), station wagons and vans and cargo liners therefore and, more particularly, to a composite/plastic truck bed liner or cargo liner with a sliding extension plate of lightweight molded composite/plastic or metal plating surrounded or not by molded composite mounted on one or more composite and/or metal rails integrally molded or attached to the structure of the bed/cargo liner.